This application claims the priority benefit of Taiwan application serial no. 91111435, filed May 29, 2002.
1. Field of Invention
The present invention relates to a tape structure and its manufacturing method. More particularly, the present invention relates to a tape structure and manufacturing method capable of preventing the formation of whiskers and recess cavities on lead surfaces.
2. Description of Related Art
In this information saturated society, multimedia applications are expanding quickly. To prepare for multimedia expansion, integrated circuit packages are increasingly digitized, networked, locally interconnected and personalized. Correspondingly, the electronic devices must have a high processing speed and serve multiple functions. In general, electronic devices are highly integrated, compact and easily mass-produced. In the past, most integrated circuit packages were fabricated using wire-bonding techniques. However, with an increase in the number of devices packed within a given package, all the devices within the package can no longer be properly linked together due to intrinsic limitations of the wire-bonding technique. Consequently, the tape automated bonding (TAB) technique has been developed. Since packages formed by the tape automated bonding (TAB) technique occupy a small volume and are relatively light and flexible, subsequent assembling and packaging are very much facilitated.
FIG. 1 is a sectional view of a conventional tape structure. The tape is formed on a film 100. The film 100 has a plurality of device holes 110. A solder mask 108 is formed on the film 100. A plurality of leads 102 is formed between the film 100 and the solder mask 108. Each lead 102 extends from the film 100 into the interior of the device hole 110. In general, the leads 102 are made from copper. The surface of each lead 102 has a metallic tin layer 106. However, the leads 102 and the film 100 as well as the leads 102 and the solder mask 108 are in direct contact and hence the junction between has no intermediate metallic tin layer 106. A tin-copper alloy layer 104 is formed between the metallic tin layer 106 and the leads 102.
FIG. 2 is a flow chart showing the steps for producing the tape as shown in FIG. 1. As shown in FIG. 2, the manufacturing process includes providing a film (200), forming the leads (202), coating a solder mask layer (204), tinning the leads (206) and baking (208).
To fabricate the tape structure, a film 100 is provided in step 200. The film 100 with a tape profile has a plurality of device holes 110 therein. Leads 102 are formed in step 202. To form the leads 102, a copper film is pressed onto the film 100 and patterned by conducting photolithographic and etching processes. A solder mask 108 is formed in step 204 by coating a layer of solder material over the film 100. The leads 102 are tinned in step 206. In the tinning operation, the exposed lead surface is electroplated to form a metallic tin layer 106. Finally, the assembly is baked in step 208 to form a tin-copper alloy layer 104 at the junction between the lead 102 and the metallic tin layer 106.
After the aforementioned tinning operation, a recess cavity may emerge on the surface of the leads 102 (details explained below) and whiskers may appear on the upper surface of the metallic tin layer 106.
FIG. 3 is a sectional view showing the emergence of a recess cavity in a conventional tape lead due to corrosion after a tinning operation. Since the edges of the solder mask 108 have a thickness smaller than the central region, the edges are more vulnerable to the corrosive attack by the tinning solution and hence are more likely to peel off from the leads 102. The peel-off area between the solder mask 108 and the lead 102 is then subjected to an oxidation-reduction reaction. In the oxidation-reduction reaction, the copper atoms on the lead 102 are oxidized into copper ions and dissolved in the tinning solution. Ultimately, a recess cavity 110 will appear on the surface of the lead 102. In the meantime, the electrons released from the copper atoms will reduce the tin ions in the tinning solution into metallic tin and adhere to the surface of the lead 102.
FIG. 4 is a sectional view of another conventional tape structure. The tape is formed on a film 300. The film 300 has a plurality of device holes 310. A solder mask 308 is formed on the film 300. A plurality of leads 302 is formed between the film 300 and the solder mask 308. Each lead 302 extends from the film 300 into the interior of the device hole 310. In general, the leads 302 are made from copper. The surface of each lead 302 has a metallic tin layer 306. However, the leads 302 and the film 300 are in direct contact and hence the junction between has no intermediate metallic tin layer 306. A tin-copper alloy layer 304 is formed between the metallic tin layer 306 and the leads 302. In other words, there is a tin-copper alloy layer (304) and a metallic tin layer (306) between the solder mask layer 308 and the lead 302.
FIG. 5 is a flow chart showing the steps for producing the tape as shown in FIG. 4. As shown in FIG. 4, the manufacturing process includes providing a film (400), forming the leads (402), tinning the leads (404), baking (406) and coating a solder mask layer (408).
To fabricate the tape structure, a film 300 is provided in step 400. The film 300 with a tape profile has a plurality of device holes 310 therein. Leads 302 are formed in step 402. To form the leads 302, a copper film is pressed onto the film 300 and patterned by conducting photolithographic and etching processes. The leads 302 are tinned in step 404. In the tinning operation, the exposed lead surface is electroplated to form a metallic tin layer 306 so that the leads 302 have a metallic tin layer thereon aside from the junction between the leads 302 and the film 300. The assembly is baked in step 406 to form a tin-copper alloy layer 304 at the junction between the lead 302 and the metallic tin layer 306. Finally, a solder mask 308 is formed in step 408 by coating a layer of solder material over the film 300.
Although the sequence of steps including lead tinning (404), baking (406) and solder mask coating is able to minimize the formation of recess cavities, whiskers still forms on the surface of the metallic tin layer 306.
FIG. 6 is a sectional view of yet another conventional tape structure. The tape is formed on a film 500. The film 500 has a plurality of device holes 510. A solder mask 508 is formed on the film 500. A plurality of leads 502 is formed between the film 500 and the solder mask 508. Each lead 502 extends from the film 500 into the interior of the device hole 510. In general, the leads 502 are made from copper. The surface of each lead 502 has a tin-copper alloy layer 504. However, the leads 502 and the film 500 are in direct contact and hence the junction between has no intermediate. tin-copper alloy layer 504. A metallic tin layer 506 is formed over the tin-copper alloy layer 504, but the junction interface between the tin-copper alloy layer 504 and the film 500 as well as between the tin-copper alloy layer 504 and the solder mask layer 508 has no metallic tin layer 506. In other words, there is a tin-copper alloy layer (504) between the lead 502 and the solder mask 508 but no metallic tin layer (506) therein.
FIG. 7 is a flow chart showing the steps for producing the tape as shown in FIG. 6. As shown in FIG. 6, the manufacturing process includes providing a film (600), forming the leads (602), conducting a first tinning operation (604), baking (606), coating a solder mask layer (608) and conducting a second tinning operation (610).
To fabricate the tape structure, a film 500 is provided in step 600. The film 500 with a tape profile has a plurality of device holes 510 therein. Leads 502 are formed in step 602. To form the leads 502, a copper film is pressed onto the film 500 and patterned by conducting photolithographic and etching processes. The leads 502 are tinned in step 604. In the first tinning operation, the exposed lead surface is electroplated to form a metallic tin layer so that the leads 502 have a metallic tin layer thereon aside from the junction between the leads 502 and the film 500. The assembly is baked in step 606 to form a tin-copper alloy layer 504 over the lead 502. A solder mask 508 is formed in step 608 by coating a layer of solder material over the film 500. Finally, a second tinning operation is carried out in step 610 to form a metallic tin layer 506 over the exposed surface of the tin-copper alloy layer 504.
The introduction of a second tinning operation in the aforementioned production process is able to minimize the probability of forming recess cavities on the leads 502 as well as whiskers on the surface of the metallic tin layer 506.
Accordingly, one object of the present invention is to provide a tape structure and fabrication method capable of preventing the formation whiskers and recess cavities on lead surfaces.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a tape structure. The tape structure comprises a film, a solder mask and a plurality of leads. The film has at least one device hole and a plurality of leads. The leads extend from the film to the interior of the device holes. The solder mask is formed on the film. The surface of the leads is covered with a tin-copper alloy layer, a tin-copper-first metal alloy layer and a metallic tin layer. The first metal includes silver, bismuth, gold, magnesium, nickel, or palladium, for example. In this invention, the tin-copper alloy layer is formed on the surface beyond the junction interface between the film and the leads. The tin-copper-first metal alloy is formed over the tin-copper alloy layer. The metallic tin layer is formed on the surface beyond the junction interface between the tin-copper-first metal alloy layer and the mask layer.
This invention also provides an alternative tape structure comprising a film, a solder mask and a plurality of leads. The film has at least one device hole and a plurality of leads. The leads extend from the film to the interior of the device holes. The solder mask is formed on the film. The surface of the leads has a copper-first metal alloy layer, a tin-copper-first metal alloy layer and a metallic tin layer. The first metal includes silver, bismuth, gold, magnesium, nickel, or palladium, for example. In this invention, the copper-first metal alloy layer is formed on the surface beyond the junction interface between the lead and the film. The tin-copper-first metal alloy layer is formed on the surface beyond the junction interface between the copper-first metal alloy layer and the solder mask layer. The metallic tin layer is formed over the tin-copper-first metal alloy layer.
In the aforementioned tape structure, the film is made from a material including polyimide and the solder mask layer is made from a material including epoxy resin, for example.
This invention also provides a method of fabricating a tape structure. A film having at least one device hole therein is provided. A plurality of leads is formed on the film. The leads are made from a material such as copper. Thereafter, a first metallic tin layer is formed on the surface of the leads (first tinning operation). A first metallic layer is formed on the surface of the first metallic tin layer. The first metallic layer is a silver, bismuth, gold, magnesium, nickel, or palladium layer, for example. A first baking operation is carried out to form a tin-copper alloy layer and a tin-copper-first metal alloy layer on the surface of the leads. A solder mask is formed over the film. A second metallic tin layer is formed on the surface of the tin-copper-first metal alloy layer. Finally, a second baking operation is carried out.
In the aforementioned method of fabricating a tape structure, the first metallic tin layer, the first metallic layer and the second metallic tin layer are formed, for example, by electroplating.
This invention also provides an alternative method of fabricating a tape structure. A film having at least one device hole therein is provided. A plurality of leads is formed on the film. The leads are made from a material such as copper. Thereafter, a first metallic layer is formed on the surface of the leads. The first metallic layer is a silver, bismuth, gold, magnesium, nickel, or palladium layer, for example. A first baking operation is carried out to form a copper-first metal alloy layer on the surface of the leads. A solder mask is formed over the film. A metallic tin layer is formed on the surface of the copper-first metal alloy layer. Finally, a second baking operation is carried out to form a tin-copper-first metal alloy layer between the copper-first metal alloy layer and the metallic tin layer.
In the aforementioned method of fabricating a tape structure, the first metallic layer and the metallic tin layer are formed, for example, by electroplating.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.